1. Field of the Invention
This invention relates generally to an apparatus for passively determining the range of a target from a submerged submarine by using two electronic periscopes.
2. Description of the Prior Art
Range finding is a process in which the distance between an object and an observer is determined. Range finding systems are either active or passive. Active systems typically involve the transmission and reception of electromagnetic energy or acoustic energy. A disadvantage of such active range finding systems is that the transmission of such electromagnetic and acoustic energy discloses the position or location of the transmitting ship.
Existing methods of using a periscope to determine a target's position by passive means are based upon visual observations that produce an accurate measurement of azimuth position but obtain only a rough estimate of target range. The range determination is made by noting how much of the operator's view the target's image fills, as measured by parallel engraved lines on a reticle in the periscope's optical path. Alternately, the range is determined by manipulation of a stadimeter embedded within the periscope. Both techniques are based upon measurement of the vertical angle subtended in the periscope by the target vessel's structure from water line to masthead. If the target's height is known, it can be related mathematically with the vertical angle of observation to yield an estimate of the distance between the submarine and the target.
A key element of this process is knowledge of the target's masthead height which initially requires target type identification. The range measurement thus obtained is only approximate and usually imposes the need to consult a document or data base to obtain the masthead height, following the procedure of target classification. The submarine's periscope may need to be raised and lowered more than once during a time consuming process of identification, height estimation, and measurement. In addition, these range measuring procedures are only effective when the target is a vessel on the ocean surface. Accurate range to an aircraft, such as a hovering helicopter, cannot be measured using the foregoing techniques because the aircraft's height above the surface is difficult or impossible to estimate.
A well known range finding concept is based upon simultaneous observation of a target from two positions where the distance of separation between the two positions is precisely known. The distance between the two observation points or optical windows is used as the base of a triangle. The angle between the baseline and the line of sight to the target is measured with precision from each of the two optical observation points, and a position triangle can be solved through knowledge of two angles and the included side. An accurate determination of range and bearing is made simultaneously, and thus the target's position is defined precisely. The greater the baseline distance, or separation between the observation points, the greater the accuracy of range calculation.
Extension of this concept to a submarine application has not been practical. A traditional submarine periscope is operated from within the control room of the ship by a single individual who turns the scope and observes its optical presentation through the eyepiece. The periscope operator effectively looks through the mast of the periscope; therefore, the periscope must be located within the pressure hull of a submarine. A submarine periscope must be narrow in order to be extended and retracted, thereby permitting only a single optical system to be included within its mast structure. A useful optical range finder requires a substantial horizontal separation between two optical systems that can be focused upon a single object simultaneously. Because of periscope limitations, submarines have not heretofore been able to use horizontal optical separation to achieve range finding capability.
To determine the range from a submarine to a target of unknown height the submarine must make multiple successive observations separated by a measured time interval during transit of the submarine from one position to another. This process, similar to a "running fix" in piloting and navigation terminology, is an attempt to simulate the desired optical separation of a range finder. It is effective only if the target is stationary.
Recent advancements in periscope technology have produced a new type of instrument that will facilitate a significant change in periscope operability. The new periscope is in the form of a remotely controlled, extendable mast, surmounted with a camera or other type of optical imaging device. The periscope operator no longer needs to be in direct contact with the periscope, since the operator observes the periscope sensor's image on an electronic display screen remote from the mast. Accordingly, the periscope mast no longer is required to pass through the control room, and the mast supporting the imaging device can be contained completely outside of the pressure hull, such as in the "sail" structure, or possibly within a ballast tank. In any case, the periscope can be controlled remotely from an operator workstation within the ship.